The present invention relates to a water purification method, in which an incoming flow of contaminated water is converted to a pressure-elevated flow which through reversed osmosis, nanofiltration, or ultrafiltration in a purification unit is divided into a purified outflow and a non-purified or contaminated outflow which, in turn, is divided into a return flow and a reject flow. The invention also relates to an arrangement for carrying out the method.
It is known in water purification processes in which reversed osmosis (RO) and ultrafiltration (UF) are employed that the quotient between the flow of purified water and contaminated water can be increased when some of the waste water is allowed to circulate through the purification process. It is also known within RO-technology and UF-technology to allow the major part of the waste water either to return to a point which lies upstream of the pressure boosting pump or to allow the water to be returned to a point downstream of that pump for circulation in a high-pressure region with the aid of a separate circulation pump.
The advantage with the first system lies in its simplicity, while the disadvantage with the system is that it requires a relatively high degree of pump work. Consequently, the first system is applied preferably on small plants which do not require the highest energy efficiency. A divided flow can be taken from the maximum nominal flow relatively easily. The second system which employs internal circulation is more complicated, since it requires an additional pump and circulation flow conduits. Although this system is more energy effective, it is more expensive to provide, because of the intricate control system required to take a divided flow from the maximum nominal flow with low energy losses.